2 Kicker comp 15s tuning help

Okay so one of these days i am going to get two 15 in kickers, either the new compS or the 2010 model comp, either way there is no difference except power handling. Now im goig to have it in a sedan trunk in a vented box about 7 cubic foot. I play a variety of music, but most importantly i want it to be able to play loudest and most efficient at really low like 20-30+ hz. The manual says the fs is 23hz so should i tune to 23 or tune slightly higher? Again i play bass boosted rap/dubstep.
Any advice and reasons why would be nice!